A widely used practice in chemical analysis is to impregnate one or more drops of liquid sample material to be examined onto a sample carrier, dry the sample carrier impregnated with the sample material, and then send the sample carrier to a laboratory for examination. The sample material to be examined can be, for example, blood and the sample carrier can be, for example, a sheet of filter paper or some other suitable material which is able to absorb the sample material. In the laboratory, one or more pieces containing the sample material to be examined are cut or punched out from the sample carrier and the piece that has been cut off is conveyed to a sample well of e.g. a microtitration plate for further analysis. The further analysis typically comprises eluting the sample material or at least part of it into sample solution in the sample well, carrying out a chemical or biochemical reaction, and subsequently carrying out an optical measurement from the sample well. The desired chemical reaction can also occur directly on the surface of the sample carrier, and the elution of the sample material is in this case not necessary.
The optical measurement can be, for example, a fluorescence measurement, a time gated fluorescence intensity measurement, a fluorescence life-time measurement, a luminescence measurement, or an absorbance measurement. The piece of the sample carrier places itself stochastically in the sample well. The stochastic location of the piece of the sample carrier in the sample well with respect to the location of the capture range of the optical measurement may influence the optical measurement result because the piece of the sample carrier may attenuate or enhance the radiation being measured. In conjunction with certain analysis methods, the piece of the sample carrier can be dark because of coloring substances, e.g. hemoglobin, which can be on the surface of the piece. In this case, the piece may disturb the optical measurement by attenuating the measured radiation even if the piece were on the bottom of the sample well. However, also in cases where the piece is white, the piece can disturb the optical measurement by typically enhancing the measured radiation.
An inconvenience related to the above described phenomenon is that it may increase the measurement deviation between replicated samples and thus it may cause additional work and additional requirements to personnel performing the optical measurements in laboratories. An optical measurement according to the prior art has to be usually taken from such a sample well that does not contain the piece of the sample carrier, i.e. the piece has been removed from the sample well or the substance to be measured has been transferred to another sample well prior to the measurement.